In a monopulse radar two, four or more separate channels transmit the same RF signal at the same time but receive ground return echoes independently. The feeds are typically horn antennas for mechanical dish antennas or individual sub-arrays for phased-array antennas. In a four channel system, each feed antenna is placed within a quadrant of the antenna. In a phase comparison (or combined amplitude and phase comparison) radar the beams from the four quadrants (A, B, C, D) are fed to a monopulse comparator where the sum and difference signals in azimuth and elevation are formed as follows:Σ=A+B+C+D Δaz=(A+B)−(C+D)Δel=(A+D)−(B+C)
The sum and difference components are fed to the receiver where they are demodulated and processed in a known manner to provide error signals to the missile guidance system.
Conventionally, the signal to be transmitted is fed to the sum channel of the comparator, divided by the comparator and supplied equally to the four antenna channels. A duplexer (transmit/receive switch or circulator and limiter) is provided in the sum channel to isolate the receiver from the transmitter whilst it is transmitting if damage to the receiver or loss of transmitter power is to be avoided. However such duplexers can cause difficulties in achieving phase and amplitude tracking between the sum and difference channels in the microwave receiver.
The present invention avoids the use of a duplexer in the sum channel.
According to the invention, there is provided a radar apparatus comprising a transmit/receive antenna having a plurality of channels, comparator means coupled to the antenna channels for providing received signal sum and difference channels at respective sum and difference ports, a radar receiver, means coupling the sum and difference ports to the radar receiver, a radar transmitter for providing a signal to be transmitted, means coupling the transmitter to a cross-channel port of the comparator means, and means for adjusting a phase length of at least one antenna channel between transmitting so that when the apparatus is transmitting, the transmitted signals from the antenna channels have a required relative phase relationship.
The adjusting means may be adapted to adjust the transmission phase length of said at least one channel relative to the transmission phase length of at least one other of said channels when the apparatus is transmitting.
In a preferred embodiment, the apparatus is a four-channel monopulse radar apparatus, the difference channels providing azimuth and elevation differences relative to a boresight of the antenna, the required phase relationship being that the transmitted signals are coherent.
However the invention is a principle applicable to larger array antennas, because the monopulse comparator is a subset of the more general Butler matrix.
The adjusting means may be adapted to switch the phase length of the at least one channel between a reception phase length and a transmission phase length depending on whether the apparatus is receiving or transmitting.
In a four-channel monopulse embodiment, two of the antenna channels may have a transmission phase length from the comparator to the antenna which is λ/4 longer than the corresponding transmission phase length of the other two channels, λ being the wavelength of the transmitted signal. Each antenna channel may comprise amplifying means controllable to provide gain, preferably high gain (e.g. 10 dB) in the transmitting direction and negligible loss (e.g. 0.5 dB) in the receiving direction.
The adjusting means may by-pass a phase length associated with the amplifying means when the apparatus is receiving.
The amplifying means may comprise a hybrid junction having two input/output ports and two further ports, each of the further ports being coupled to a respective amplifier via a respective said phase length, the adjusting means being configured selectively to cause reflection of a signal through the hybrid junction from one of the input/output ports to the other when the apparatus is receiving.
Thus the adjusting means may comprise, in parallel with each amplifier and its phase length, a controllable resistance device having a first bias condition in which it constitutes an open circuit and a second bias condition in which it constitutes a short circuit and causes reflection of said signal through said hybrid junction from one of said input/output ports to the other, said first and second bias conditions corresponding to radar transmission and reception respectively.
Each said amplifier may be an IMPATT diode or other injection locked oscillator.
Each controllable resistance device may be a PIN diode.
Each amplifying means may be positioned within its antenna channel such that the total reverse transmitter leakage into the antenna channels which appears in the sum and difference channels is substantially nil. Thus the amplifying means in two of the antenna channels of a four-channel monopulse apparatus may be λ/4 closer to the antenna than the amplifying means of the other two channels.
A further aspect of the invention provides an amplifier/duplexer for an antenna channel of a radar apparatus, comprising a first hybrid junction having two input/output ports for connection into the antenna channel, and two further ports each connected to a parallel combination comprising a transmission signal amplifier and switching means switchable between a transmit condition in which an amplified signal for transmission is passed to the antenna channel and a receive condition in which a signal received in the antenna channel is applied to one of the input/output ports and reflected to the other input/output characterised by each amplifier being configured to receive the signal for transmission other than via the sum port of a comparator and by a phase length adjuster which is disposed in the path of the transmission signal so that the amplified signal for transmission has a required phase.
As noted above the phase adjuster may be an additional phase length (e.g. λ/4) between the switching means and the transmission signal amplifier.